Method and apparatus for determining the position of a feature of an object

ABSTRACT

A system for locating a feature of an object disposed in a frame includes a spot projector for projecting a spot in the general vicinity of the feature and an imaging device with a limited field of view. The spot projector is a precision device which projects the spot towards a preselected location on the frame. When the spot and the feature are in the field of view of the imaging device, the absolute location of the feature is determined from its selective distance to the spot. Provisions are also made for determining depth or relief information about the object.

BACKGROUND OF THE INVENTION

a. Field of Invention

This invention discloses a method and device for determining the exactposition of an object within a field of view, for the operation ofrobots.

b. Description of the Prior Art

There is currently a strong impetus to design and manufacture robotscapable of complex manipulations of various objects. A prime requisitefor such a robot is accurate information regarding the exact position ofthe object being manipulated within a reference frame because withoutthis information the robot cannot perform its designated tasks. Encodersystems integral with the robot mechanisms and various imaging devicessuch as video cameras have been used for this purpose. Both approacheshave limitations. For example, the encoder approach is subject to errorsdue to loading of the robot structure. Also, in video approachesfrequently there is a trade-off between the available field of view andresolution. Thus, for example, a video camera with a capability toproduce 512×512 pixels and a resolution of 0.002" has a one inch x oneinch field of view.

OBJECTIVES AND SUMMRY OF THE INVENTION

An objective of the present invention is to provide a method ofdetermining the position of a feature of an object by repositioning avideo camera thereby eliminating the need of several cameras.

Another objective is to provide a single position determining methodwhich may be easily incorporated into several robotic operations such asqualifying (or recognizing) parts, qualifying particular features on apart, locating a base sheet on a fixture, locating subassembly parts ona base sheet and locating an assembly for riveting.

A further objective is to provide a method which may be easily adoptedto three dimensional analysis.

Other objectives and advantages of the invention shall become apparentfrom the following description of the invention.

In accordance with the present invention, a video camera is positionedwith its field of view generally covering a part or a feature of a partdisposed on a reference plane. A source of light, preferably a laser, isalso directed to form a light pattern on the part within the field ofview of the camera. For simplicity of description this light patternwill also be referred to hereinafter by the more specific term "spot."The exact position of the spot is known with great accuracy, andtherefore the absolute position of the part or its feature is determinedby measuring the relative distance between the part to the spot as seenby the camera.

A second laser spot from a different angle may be used to makemeasurements when the first laser is occluded. The two laser sources mayalternately be used to derive the depth (or profile) of the part.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the elements of a preferred embodiment of an apparatus fordetermining the position of an object constructed in accordance with theinvention;

FIG. 2 shows how the apparatus of FIG. 1 is calibrated;

FIG. 3 shows the field of view of a video camera which is part of theapparatus of FIG. 1;

FIG. 4 shows an alternate embodiment of the invention; and

FIGS. 5A, 5B, 5C show the field of view for the camera of FIG. 4 with arelatively flat, a concave and a convex object respectively.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, an object 10 is secured to frame 12 to determine the exactlocation of a particular feature of the object. For this purpose thereis provided a video camera 14 mounted on a robotic arm 16 adapted topoint the camera toward any portion of the surface 12 under thedirection of a microprocessor-based control panel 18. (The electricalconnections to the control panel are omitted for the sake of clarity.)Control elements are generally available which can direct the cameratowards any portion of the frame 12 with an accuracy of about 1/4 inch.In FIG. 1 camera 20 is pointed so that it can view a rectangular portion20 of the object 10.

There is also provided a pedestal 22 which carries a laser 24 and twopositioning mirrors 26 and 28. Mirror 26 controls the vertical angle ofthe beam generated by laser 24 (i.e. its elevation) while mirror 28controls the horizontal angle of the beam (i.e. its azimuth). The twomirrors are also controlled by the control panel 18. As shown in FIG. 1laser beam 30 forms a laser spot within the field of view 20. The laserspot is used as a relative fiducial or reference and its exact positionmust be known before hand. Therefore before the system is used, thelaser mirrors 26, 28 are calibrated as follows.

After the surface 12, pedestal 22 and robot 16 are securely mounted, aself-calibration is performed. For this purpose, the frame is providedwith a plurality of targets 32, 34, 36 and 38 positioned very preciselyon the frame. A movable, two dimensional target is also used. Thetargets may comprise, for example, a plurality of two-dimensionalphotosensors such as the Hamamatsu PSD 1200. Alternatively, the targetsmay be high resolution charged coupled device (CCD) sensor arrays. Eachtarget is coupled to control panel 18. The baseline, 40, of the laserbeam is found just by interactively positioning the movable targetwithin, and parallel with, the frame. When equal changes of beam anglefrom a center location produces equal beam displacements on the target,along both axes, then the baseline, 40, is the center location. Thelaser beam 30 is then directed at target 32 (as shown in FIG. 2) and thetwo mirror positions are finely adjusted to center the laser beam on thetarget. The azimuth angle A₁ and vertical angle V₁ are also determinedwith respect to a baseline 40, perpendicular to frame 12 which impingesthe frame at 0, and a horizontal plane containing said line 40. Next thebeam 30 is directed at a second target 34 and the corresponding anglesA₂ and V₂ are determined with respect to the baseline 40. If the axes ofmirrors 26, 28 and targets 32 and 34 coincide, V₁ is equal to V₂ and thelength Z of baseline 40, may be determined by the equation: ##EQU1##where W is the spacing between targets 32 and 34. Otherwise the anglesto target 36 are determined and Z is then calculated by using simpletrigonometry. The results may be confirmed by using the anglemeasurements obtained by directing the laser beam to fourth target 38.

Thus, after the pedestal 22, robot 16 and frame 12 are securelypositioned, the self-calibration steps described above determine theexact location of origin 0 and the length Z of baseline 40.

In FIG. 3, the field of view 20 is shown as seen by camera 14. In thisfield there is visible a portion of object 10. The object has onefeature 42, one feature 44 and a laser spot 46 formed by laser beam 30as described above. If the spot has diverged somewhat, camera 14 isequipped to determine its center. The position of key features of thelight pattern such as the center of a spot in the field of view can bedetermined through image processing intensity, centroid processing, orany other suitable means. The position of point 46 with respect to 0 iseasily determined from laser beam pointing angles A & V. For example, ifmirrors 26 and 28 direct beam 30 at angles of V₃ and A₃ respectively,then

    X=Z tan A.sub.3

    Y=Z tan V.sub.3

where X and Y are respectively the horizontal and vertical coordinatesof point 46 with respect to 0.

From this information, the exact position of features 42 and 44 iseasily derived by measuring the distance to point 46, for example, bycounting the number of pixels in both the horizontal and verticaldirection between the center of feature 42 and spot 46. The exactposition of feature 42 is these resolved distances added to the X, Yposition of spot 46.

Thus, the position of feature 42 or 44 are determined by projecting aknown reference (spot 46) into the field of view 20, determining therelative difference between the feature and the reference, and addingthis difference to the reference.

Once the position of an object is determined with respect to frame 12, awork robot (not shown) remotely controlled from the control panel may beused to shift the object to a preselected location. Since the work robotmay occlude the beam from the laser 24, a second laser 70 may beinstalled on the other side of camera 14 as shown in FIG. 4. Laser 70with its mirrors 72, 74 is calibrated in the same way as laser 24, usingthe same targets so that a common reference frame is established forboth lasers. With this configuration, the position of object 10 in frame12 may be determined by using either laser 24 or 70.

In addition, a two-laser system may also be used to determine variationsin depth on object 10. During calibration the location of the spots aredetermined on the calibration surface 12. Conveniently, for example, thespots can be made to substantially coincide on such calibration surface12. In such case when the surface of object 10 is in the same plane asthe calibration surface, then the spots will also substantially coincideon the object 10, except for effects of random errors in the laseraiming system. This is illustrated in FIG. 5A which shows laser spot 46from laser 24 and laser spot 76 from laser 70 and their centroids 46'and 76' all with only minimal separation.

When in some areas the surface of object 10 is protruding beyond thecalibration plane, the spots will separate, with the spot of the leftlaser moving to the left and the spot of the right laser moving to theright. This is illustrated in FIG. 5B. When the surface of object 10 isfarther away from the calibration plane, then the laser spots will alsoseparate, but in this case the spot from the left laser moves to theright and the spot from the right laser moves to the left, asillustrated in FIG. 5C.

The separation between the spots is proportional to the depth variationsof the object 10. To identify the individual laser spots, one of thelaser beams may be blocked momentarily. Using the methodology describedabove the profile of the object 10 can be mapped.

Obviously, numerous modifications may be made to the invention withoutdeparting from its scope as defined in the appended claims.

What is claimed is:
 1. A method of determining the position of a featureon an object disposed in a frame comprising the steps of:directing animaging device with a field of vision towards the feature; projecting arelative reference light pattern into the field of vision, saidreference light pattern being disposed at a known location with respectto a fixed reference; and measuring the distance between said relativereference light pattern and said feature as indicated by said imagingdevice.
 2. The method of claim 1 further comprising providingcalibrating targets on said frame, for calibrating said reference lightpattern.
 3. A device for determining the position of a feature on anobject comprising:a frame for holding the object; imaging means having afield of view; light pattern projecting means for projecting a patternor spot to a known location on said field of view; and control means fordirecting said imaging means towards said field of view andsimultaneously directing said light pattern projecting means to projectsaid light pattern within said field of view; whereby the position ofthe feature with respect to the frame is determined by the position ofthe light pattern and the position of the feature relative to said lightpattern.
 4. The device of claim 3 wherein said imaging means comprises avideo camera.
 5. The device of claim 3 wherein said light patternprojecting means comprises a laser and means for directing the laserbeam along two axes across the frame.
 6. The device of claim 5 whereinthe frame comprises a plurality of targets disposed at a preselecteddistance, and said control means is further provided to calibrate thelaser by directing said laser beam to said targets.
 7. The device ofclaim 6 wherein said targets comprise an array of photosensitiveelements coupled to said control means said elements being activated bysaid light pattern when the laser beam is directed toward thecorresponding target.
 8. The device of claim 5 wherein said lightpattern generating means comprises a second laser for generating a laserbeam, and second means for directing said second laser beam toward saidframe.
 9. The device of claim 5 wherein said means for directing thelaser beam comprises two mirrors for directing the beam in twoorthogical directions.
 10. The device of claim 5, wherein said means fordirecting the laser beam comprises one mirror mounted on two orthogonalaxes for directing the beam in two orthogical directions.